The present invention relates to novel biodegradable blocks for models, which are made of a biodegradable plastic material comprising a biodegradable base polymer, especially that consisting of a starch ester or a mixture of a starch ester and a cellulose ester.
The term model referred to herein means a so-called patterning model to be used for producing final products, which includes, for example, prototypes, master models, styling models, design models, foundry patterns, crafting models, copying models, and models for confirming tapes in NC machining systems.
The degree of substitution (DS) referred to herein indicates a degree of esterification of polymer ester, which is represented by an average value (at most DS of being up to 3) of substituted hydroxyl groups per one glucose residue in a polymer ester. This DS value is determined by the method referred to the paper as follows; R. L. Whistler, ed., xe2x80x9cMethod in Carbohydrate Chemistryxe2x80x9d, Vol. III-Cellulose, Academic Press, Inc., New York, p. 201-203, (1964)
Recently, models such as those mentioned above are being made of plastic block, in place of conventional wood and the like, since plastic blocks do not require any care for grain orientation, hygroscopicity and other matters. To produce such models, in general, plastic blocks are cut or machined with hand tools, such as chisels, planes, saws, files and rasps, or with machine tools such as NC machines and lathes.
For their necessary characteristics, plastic blocks for such models are required to have good workability and machinability and also predetermined mechanical properties (e.g., dimension stability, heat resistance, strength). It has heretofore been said that nothing but thermosetting plastics could satisfy such characteristics with ease, for example, used are unsaturated polyester resins, phenolic resins, epoxy resins and polyurethane resins.
On the other hand, recently, there is increasing a demand for biodegradable plastic, especially for those of industrial waste, which can be recycled or which do not pollute the environment when discarded, from the viewpoint of saving natural resources and of environmental protection.
In general, thermosetting plastics to be used for the models, being different from thermoplastic plastics, are not plasticized even under heat but could be only decomposed under heat, and it is difficult to recover and recycle them.
Therefore, at present, the models made of such thermosetting plastics are, after having been used to be useless, discarded as industrial wastes, thus ending up in incinerators or landfills.
However, where these are incinerated, they give high combustion heat to damage incinerators and, in addition, often generate much soot and smoke, as being made of thermosetting plastics. If, on the other hand, these are desired to be in landfills, the spaces for landfills are insufficient and, in addition, they do not decompose almost semi-permanently to eventually have some negative influences on soil organisms, etc.
Given the situation, biodegradable plastics are now desired which are free from the problems with plastics, especially thermoplastic plastics to be discarded. Biodegradable plastics can be composted in kitchen garbage processors or composters. In addition, even when discarded in landfills, they are decomposed in soil without having almost no negative influence on soil organisms and others in landfills.
Biodegradable plastics that are at present commercially available include, for example, polyesters to be produced by microorganisms (e.g., polyhydroxybutyrate valerate), and synthetic plastics, such as linear aliphatic polyesters, polycaprolactones, polylactic acids and starch-polyvinyl alcohol alloys.
However, it is said that many such commercially-available biodegradable plastics are unfavorable for use in blocks for models because of their poor physical properties. Specifically, their workability or machinability with hand tools or machine tools is poor. In other words, such biodegradable plastics are too much adhesive to cutting tools and are difficult to cut and work with hand tools; and, if they are machined with machine tools, their cut surfaces are partly melted, and it is difficult to obtain products with well cut surfaces (well finished surfaces).
For these reasons, the development of biodegradable plastics with both good workability and machinability and good mechanical and physical properties that are suitable for use in blocks for models, while naturally having good biodegradability, has been desired in the art.
In view of the above, the object of the patent invention is to provide novel biodegradable blocks for models, which have good workability and machinability and which have good mechanical characteristics suitable for models, while naturally having good-biodegradability.
(1) We, the present inventors have assiduously studied in order to attain the above-mentioned object, and have found that shaped products of esterified starches can be easily cut with hand tools such as knives. On the basis of this finding, we have further studied and, as a result, have further found that a biodegradable thermoplastic composition comprising an esterified starch having a particular degree of substitution with particular substituents or a mixture of such an esterified starch and an esterified cellulose having a particular degree of substitution with particular substituents has good workability and machinability and good mechanical properties (especially, good dimension stability, high heat resistance) suitable for use in blocks for models, resulting in the finding of biodegradable blocks for models, which comprise the component mentioned below.
Specifically, the present invention provides a biodegradable block for models which is made of a biodegradable plastic material comprising, as the base polymer, a biodegradable polymer, and which is characterized in that:
said biodegradable polymer is a mixture comprising (a) an esterified starch having a DS of about 0.4 or more and (b) an esterified cellulose having a DS of about 0.4 or more, in a ratio by weight, (a)/(b), of being from 10/0 to 1/9.
Wherein said esterified starch basically contains one or more members selected from the group as follows:
(1) An vinyl esterified starch prepared by esterifying a starch with an esterifying reagent of a vinyl ester in a non-aqueous organic solvent in the presence of an esterification catalyst.
(2) An esterified, polyester-grafted starch which is formed by esterifying starch and grafting starch with polyester.
(3) A mixed esterified starch of which hydrogen in the reactive hydroxyl group of the same starch molecule is substituted with an acyl group that has 2 to 4 carbon atoms (short chain acyl group) and an acyl group that has 6 to 18 carbon atoms (long chain acyl group).
In addition, wherein said esterified starch contains a starch ester having a DS of about 1.0 to 2.8, which is produced by a process wherein a purified starch containing at least 50% of amylose is reacted with an acylation reagent in the presence of a basic catalyst in an anhydrous aprotic solvent.
Esterified starch have been reported in literature, but as far as we, the present inventors know, there is no report referring to the use of esterified starches as thermoplastics to give commercial product.
On the other hand, of esterified cellulose, cellulose acetate and cellulose acetate propionate are much used as plastic materials for producing shaped articles and films.
We have already proposed that esterified starches and mixtures of esterified starches and esterified cellulose are both usable as biodegradable plastic materials, in our own prior patent applications (see Japanese PCT Patent Application Laid-Open No. 8-507101, Japanese Patent Application Laid-Open Nos. 8-188601 and 8-143710, and Japanese Patent No. 2742892).
However, in the above-mentioned patent applications, we expected the use of the materials for shaped articles, such as containers, dishes, cups, knives, forks and spoons, and the use thereof in horticultural and agricultural use, for example, as films, sheets, laminates and foams. In these, we did not expected them to be applicable to blocks for models that require the workability, machinability and dimension stability such as those mentioned hereinabove.
(2) In preferred embodiments of the present invention, the esterified starch has a DS of from about 1.0 to about 2.8, the esterified starch is prepared from a high-amylose starch having an amylose content of about 50% or higher, and it has ester groups with from 2 to 18 carbon atoms.
In more preferred embodiment, the esterified starch is starch acetate, starch propionate, starch acetate propionate, or a mixture of these.
A plasticizer may be added to the biodegradable polymer for the purpose to improve the shapability, workability and machinability of the block. Preferably, an ester plasticizer miscible with the esterified starch and the esterified cellulose may be added thereto in an amount of up to 35% by weight.
In order to further improve the mechanical properties of the block, an inorganic or organic reinforcing filler may be added to the biodegradable plastic material in an amount of up to about 50% by weight.
The biodegradable block for models of the present invention can be produced by kneading the biodegradable polymer optionally along with such an plasticizer and a reinforcing filler, forming it into biodegradable plastic pellets or others, putting them into a mold, melting them under heat therein, and shaping the resulting melt into a block through press shaping or the like.
To produce biodegradable blocks from the block of the invention, the block may be cut or machined with hand tools or machine tools.
The biodegradable block for models of the present invention, as having the constitution mentioned hereinabove, has good workability and machinability and also have necessary mechanical properties (especially, good dimension stability and high heat resistance). as will be demonstrated in the example to be mentioned hereinunder.
The results of the present invention that produces the biodegradable block for models having good workability and machinability and even good mechanical properties are surprising, as will be demonstrated in the cutting and machining tests mentioned hereinunder, in which samples of the biodegradable block of the invention were compared with samples of conventional biodegradable plastics such as those mentioned hereinabove, resulting in that the workability and the machinability and even the mechanical properties of the conventional biodegradable plastic samples were inferior to those of thermosetting plastic samples.